The amount of aggregate for asphalt concrete mix is usually 90 to 95 percent by weight and 75 to 85 percent by volume. Aggregates are primarily responsible for the load bearing capacity of a pavement. Aggregate is defined as any inert mineral material used for mixing into graduated particles or fragments. It includes sand, gravel, crushed stone, slag, screening and mineral fillers.
source of aggregate
Aggregates for asphalt concrete are usually classified according to their source or means of preparation. they include:
(1) pit aggregate – Gravel and sand are natural aggregates and are usually potted materials.
(2) processed aggregates – Natural gravel or stone that has been crushed and screened are specific processed aggregates. Stone dust is also generated in the crushing work.
(3) Synthetic or synthetic aggregates – Aggregates resulting from modification of a material, which may involve both physical and chemical changes, are called synthetic or synthetic aggregates. Blast furnace slag is the most commonly used artificial aggregate or light aggregate.
The selection of a composite material for use in asphalt concrete depends on the availability, cost and quality of the material, as well as the type of construction.
The suitability of aggregates for use in asphalt concrete is determined by evaluating the material:
(1) sizing and grading
The maximum size of the aggregate specifies the size of the smallest sieve through which 100% of the material will pass. The grading of an aggregate is determined by sieve analysis. Maximum size and grading are always governed by specifications that determine the particle size distribution to be used for a particular composite material for asphalt mixing. The particle size distribution determines the consistency and density of the asphalt mixture.
Read also: Sieve analysis test of aggregates
Some aggregates contain foreign or harmful substances that make them undesirable for asphalt concrete mixes. (Example: earthen lump, shale, organic matter, etc.). To detect the presence of harmful materials in the overall context IS: 2386 – Part-2,
The sand-equivalent test, described in Ashto T 176, No. 4 (4.75 mm) is a method for determining the relative proportion of materials such as harmful fine dust or clay to the total portion passing through the sieve.
Read also: Testing for friable particles in lumps and aggregates
Read also: Determination of light weight pieces in aggregates
Read also: Sand Equivalent Value Test of Fine Aggregates
Aggregates are subjected to additional crushing and abrasive wear during the manufacture, placement and compaction of asphalt concrete mix. Aggregates are also subject to friction under traffic loads. They must demonstrate the ability to resist crushing, fall and disintegration. The Los Angeles Abrasion Test measures wear resistance of aggregates.
Read also: Los Angeles abrasion test of aggregates
Also read: Crushing Value Test of Aggregates
Also read: Impact Value Testing Procedure Of Aggregates
The aggregate for asphalt concrete paving must be durable. They should not deteriorate or decompose under the action of weather. The objects considered under weathering are freezing, thawing and changes in moisture content and changes in temperature. Toughness test is an indication of resistance to weathering of finer and coarser aggregates. Look for Testing Procedures Ashto T 104 or IS: 2386 Part-V.
Also read: Strength Testing Procedures Of Aggregates
(5) Particle size (flat and elongated or F/E)
The particle size alters the workability of the mixture as well as the compaction effort required to obtain the required density. The particle size also has an effect on the strength of the asphalt concrete mix. Irregular or angular particles when compressed tend to stick together and oppose displacement.
Read also: Flexibility index value test of aggregates
Read also: Elongation index value test of aggregates
(6) Surface texture (coarse gross angularity (CAA) and fine gross angularity (FAA))
Like particle size, surface texture also affects the workability and strength of an asphalt concrete mix. Surface texture is often considered to be more important than the size of the aggregate particles. The texture of a rough, sandpaper-like surface as opposed to a smooth one increases the strength of the mixture.
Read also: Angularity number test of sets
The porosity of an aggregate is usually indicated by the amount of water absorbed when it is soaked in water. A certain degree of porosity is desirable, as it allows the aggregate to absorb the binder, which then forms a mechanical bond between the binder film and the stone particle.
Read also: Specific gravity and water absorption test procedure of aggregates
(8) affinity for binder
Separation (separation) of the binder film from the aggregate through the action of water can make the composite material unsuitable for mixing asphalt concrete. Such a material is called hydrophilic (water-loving). Many of these materials can be used with the addition of heat stabilizing additives that reduce the stripping action. Aggregates that exhibit a high degree of resistance to stripping in the presence of water are generally best suited in asphalt concrete mixes. Such aggregates are said to be hydrophobic (water haters). Why hydrophobic or hydrophilic aggregates behave as if they are is not fully understood. Explanation is not so much important as the ability to avoid the use of congruent aggregates to detect and isolate properties.
The loss of strength resulting from damage caused by “stripping” under laboratory controlled accelerated water conditioning is determined according to Ashto T 283 or according to IS: 6241-1971.